Gas turbines for vehicles or stationary plants are customarily equipped with a compressor, which draws ambient air and raises its pressure to a relatively high level in accordance with the design requirements of the combustion process of the gas turbine. The compressed air is then directed into a combustor, mixed with fuel therein and ignited. The compressed high-energy combustion gases from the combustor flow into the turbine, where they are expanded, performing mechanical work. Axial or radial compressors are used.
While the temperature of the air raises, when compressed, in the air intake system of the compressor the pressure firstly drops and then increases behind the first compressor stage. From this reason, at particular low ambient temperature the dew point temperature of air may be reached and on the surfaces of the intake structures ice forms itself from the condensing of moisture. This ice-forming mechanism usually occurs in cold regions of the world. Ice mainly forms on the stator parts of the compressor intake (bellmouth), on the inlet guide vanes (IGV) and partly on the rotor blades of the first stage. Behind the first compressor stage air temperature rapidly increases, thus protecting the components of the downstream compressor rows from ice formation. The presence of ice on the surface of these structures leads to an additional pressure drop, what results into a performance loss of the gas turbine plant. In a worse case, pieces of ice detach from the surface, e.g. of the inlet guide vanes and cause damages, particularly with the rotor blades of the first compressor stage.
Hence there is a need for a solution of effectively suppressing the formation of ice on said structure elements or, if ice deposits already have been formed on the surface, to remove those ice deposits and to prevent their reformation.
Various systems for suppressing ice-formation or such for removing ice deposits are known in the technical field of compressor operation.
The dew point temperatures of gas-vapor-mixtures are available from psychrometric charts. Following these data two types of sensors are usually applied that either monitor the condition of ice creation or detect ice building-up.
A known solution for protecting the external surfaces of the structures at the compressor intake from icing, practically used in the field of stationary gas turbine power plants, comprise a compressor bleed anti-icing system (i.e. to bleed hot air from the compressor and convey it to the intake area for heating) or applying a hot water heat exchanger system or arranging resistive heating elements to a stator wall in the intake section.
However, the drawbacks of those systems are a reduction of the performance of the gas turbine (losses up to 0.3%), the requirement of additional investment costs, special monitoring systems, costly maintainability and a limited applicability. In field experience the building-up process of ice on the first compressor stage can take a few seconds before the anti-icing system starts to operate effectively.
U.S. Pat. No. 4,732,351 discloses a device for suppressing the formation of ice on the external surface of various articles, particularly aircraft structures. According to this solution a piezoelectric material is applied to the relevant surface, which has to be kept ice free. An available power source is adapted so that, whatever the source, it is converted into an alternating current. A microprocessor is provided that, in accordance with the relevant parameters of the material used, a current of proper amplitude, duration, wave length and shape is received, which will realize the amount of movement for deformation of the piezoelectric material necessary to prevent ice formation. This ice-suppression or ice-removing mechanism is based on the forced vibration of the piezoelectric material itself. This device requires electrical power adhered to a target structure. The electrical power is conventionally supplied via electric wires.
It is a well-known phenomenon that during operation of a compressor synchronous and asynchronous vibrations of the rotating blades are generated due to aerodynamic effects. Resonances in the blade may lead to significant problems like vibratory crack formation which constitutes a critical type of failure for compressor blades.
CH 704127 discloses a solution of damping blade vibrations in turbomachines by using the piezoelectric effect. This solution is based on the concept of converting the mechanical vibratory energy of the vibrating blade into electrical energy and afterwards to convert this generated electrical energy into heat loss. And this first step of converting mechanical energy into electrical energy is realized by using the piezoelectric effect, namely by firm installation of at least one piezoelectric element into the blade to be damped. As a result of blade vibrations the piezoelectric element is deformed, thereby generating an electrical voltage. By integrating said element into an electrical network with an ohmic resistor the generated electric voltage induces an electrical current producing ohmic heat loss in the connected network.